This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:    3GPP third generation partnership project    DL downlink (eNB towards UE)    DwPTS downlink pilot time slot    eNB EUTRAN Node B (evolved Node B)    EPC evolved packet core    EUTRAN evolved UTRAN (LTE)    FDD frequency division duplex    FDMA frequency division multiple access    GP guard period    LTE long term evolution    MAC medium access control    MM/MME mobility management/mobility management entity    Node B base station    OFDMA orthogonal frequency division multiple access    O&M operations and maintenance    PDCP packet data convergence protocol    PDCCH physical downlink control channel    PDSCH physical downlink shared channel    PHY physical (layer 1)    PRACH physical random access channel    RA-RNTI random access radio network temporary identity    RACH random access channel    RLC radio link control    RRC radio resource control    SGW serving gateway    SC-FDMA single carrier, frequency division multiple access    TDD time division duplex    T-CRNTI temporary cell random access radio network temporary identity    TTI transmission timing interval    UE user equipment    UL uplink (UE towards eNB)    UpPTS uplink pilot time slot    UTRAN universal terrestrial radio access network
The specification of a communication system known as evolved UTRAN (EUTRAN, also referred to as UTRANLTE or as EUTRA) is currently nearing completion within the 3GPP. As specified the DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.7.0 (2008-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Access Network (EUTRAN); Overall description; Stage 2 (Release 8), incorporated by reference herein in its entirety. This system may be referred to for convenience as LTE Rel-8, or simply as Rel-8. In general, the set of specifications given generally as 3GPP TS 36.xyz (e.g., 36.211, 36.311, 36.312, etc.) may be seen as describing the entire Release 8 LTE system.
FIG. 1A reproduces FIG. 4.1 of 3GPP TS 36.300, and shows the overall architecture of the EUTRAN system. The EUTRAN system includes eNBs, providing the EUTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1 MME interface and to a Serving Gateway (SGW) by means of a S1 interface. The S1 interface supports a many to many relationship between MMEs/Serving Gateways and eNBs.
The eNB hosts the following functions:    functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);    IP header compression and encryption of the user data stream;    selection of a MME at UE attachment;    routing of User Plane data towards Serving Gateway;    scheduling and transmission of paging messages (originated from the MME);    scheduling and transmission of broadcast information (originated from the MME or O&M); and    a measurement and a measurement reporting configuration for use in mobility and scheduling.
In the present LTE system preamble responses are sent utilizing both the PDCCH and the PDSCH. Each RACH resource (time and frequency resource reserved for preamble transmission) is associated with a RA-RNTI (random access radio network temporary identity). When the base station (eNB) observes a preamble, it transmits the preamble response on the PDSCH on a resource that is indicated by a PDCCH addressed with the RA-RNTI. More specifically, when a Random Access Response message is transmitted, the CRC word of the corresponding PDCCH is masked by RA-RNTI. When searching a preamble response the UE tries to find a RA-RNTI masking corresponding to the frequency and time resource that the UE had used when sending its preamble. In this manner the preamble response on the PDSCH is unambiguously associated with preambles transmitted on a certain time-frequency resource.
The system is flexible in the sense that the base station can acknowledge in the same PDSCH message several preambles that have been transmitted in the same RACH resource, but that carry different signatures (preamble sequences). In addition, the responses can be sent in a time window that is configurable up to a duration of 10 ms.
In the present LTE system the responses to a set of UEs that listen to the same RA-RNTI can be combined into the same message. However, responses corresponding to different RA-RNTI cannot be combined, and PDCCH and PDSCH messages must be sent separately for each RA-RNTI (i.e., each RACH time-frequency resource). Considering the limited PDCCH resources this is not an efficient procedure. Because the base station does not know the channel state of the UEs, a PDCCH entry for a preamble response must be heavily coded, which consumes significant PDCCH resources. This can lead to problems, especially in the TDD system of LTE where several PRACH resources can exist in one subframe, and where random access responses cannot be distributed in time as flexibly as in the FDD system. This is true at least for the reason that in the TDD system there are gaps in the PDCCH due to subframes reserved for UL.
One LTE specification of interest herein is 3GPP TS 36.211 V8.5.0 (2008-12) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8). As is stated in subclause 4.2, the frame structure type 2 is applicable to TDD.
The PRACH is described in subclause 5.7 of 3GPP TS 36.211 V8.5.0.
Another LTE specification of interest herein is 3GPP TS 36.321 V8.5.0 (2009-03) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (EUTRA); Medium Access Control (MAC) protocol definition (Release 8). The specification describes in subclause 5.1 the overall Random Access procedure followed by the UE, in subclause 5.1.3 the Random Access Preamble transmission, and in subclause 5.1.4 the Random Access Response reception.
For example, as currently specified for Rel-8 in subclause 5.1.4 “Random Access Response reception”, once the Random Access Preamble is transmitted and regardless of the possible occurrence of a measurement gap, the UE shall monitor the PDCCH for Random Access Response(s) identified by the RA-RNTI defined below, in the RA Response window which starts at the subframe that contains the end of the preamble transmission plus three subframes and has length ra-ResponseWindowSize subframes. The RA-RNTI associated with the PRACH in which the Random Access Preamble is transmitted, is computed as:RA-RNTI=1+t—id+10*f—id, where t_id is the index of the first subframe of the specified PRACH (0≦t_id<10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0≦f_id<6). The UE may stop monitoring for Random Access Response(s) after successful reception of a Random Access Response containing Random Access Preamble identifiers that matches the transmitted Random Access Preamble.
It is further specified in subclause 5.1.4 that if a downlink assignment for this TTI has been received on the PDCCH for the RA-RNTI, and the received TB is successfully decoded, the UE shall regardless of the possible occurrence of a measurement gap: if the Random Access Response contains a Backoff Indicator subheader:                set the backoff parameter value in the UE as indicated by the BI field of the Backoff Indicator subheader and Table 7.2-1,        else, set the backoff parameter value in the UE to 0 ms.        
If the Random Access Response contains a Random Access Preamble identifier corresponding to the transmitted Random Access Preamble (see subclause 5.1.3), the UE shall consider this Random Access Response reception successful and process the received Timing Advance Command (see subclause 5.2) and indicate the preambleInitialReceivedTargetPower and the amount of power ramping applied to the latest preamble transmission to lower layers (i.e., (PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep); process the received UL grant value and indicate it to the lower layers; if ra-PreambleIndex was explicitly signaled and it was not 000000 (i.e., not selected by MAC) consider the Random Access procedure successfully completed.
If no Random Access Response is received within the RA Response window, or if none of all received Random Access Responses contains a Random Access Preamble identifier corresponding to the transmitted Random Access Preamble, the Random Access Response reception is considered not successful and the UE shall, among other activities, if in this Random Access procedure the Random Access Preamble was selected by MAC: based on the backoff parameter in the UE, select a random backoff time according to a uniform distribution between 0 and the Backoff Parameter Value; delay the subsequent Random Access transmission by the backoff time; and proceed to the selection of a Random Access Resource (see subclause 5.1.2).
Of particular interest herein are the further releases of 3GPP LTE targeted towards future IMT-A systems, referred to herein for convenience simply as LTE-Advanced (LTE-A).
Reference can be made to 3GPP TR 36.913, V8.0.1 (2009-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for Further Advancements for E-UTRA (LTE-Advanced) (Release 8), incorporated by reference herein in its entirety. One element of the LTE-A system is the proposed use of the UHF band (698-960 MHz, referred to simply as 900 MHz) and a 2.3 GHz band (referred to simply as 2 GHz).